I have a custom, globally-scoped function in my Express app, foo. When running my Jest test scripts, this function is caught as undefined. Thus, any tests using them fail.
index.d.ts:
declare global{
function foo(): string;
}
export {};
src/Utils/index.ts:
global.foo = function foo(){
return "bar";
};
src/Modules/Example.module.ts:
export const test = () => {
// This will return bar, as expected, when developing.
// A reference error will only be thrown when running npm test.
return foo();
};
src/Modules/Example.test.ts:
import { test } from "./Example.module";
describe("modules/example", () => {
describe("test", () => {
it("returns bar", () => {
let bar = test();
expect(bar).toBe("bar");
});
});
});
Despite this not being an issue while developing, this test results in the error:
ReferenceError: foo is not defined.
export const test = () => {
return foo();
^
...
};
You can specify src/Utils/index.ts as a setup file, which Jest will load and execute before running tests. You can add it to your Jest configuration file (or create one if you don't have one):
Assuming a CJS-format Jest configuration, jest.config.js:
module.exports = {
// Your other configuration options
"setupFiles": ["<rootDir>/src/Utils/index.ts"]
};
It will look slightly different if you are using a JSON or TypeScript Jest configuration file.
However I don't recommend using global variables (even if you use them a lot). With a proper code editor setup, it is easy to import a function from another file.
I have a function that returns true or false, lets call it myFunc
myFunc (){
if(something){return true}
else{return false}
}
that's what it does for sake of arg
I then call it somewhere else
if(myFunc()){
}else{
}
when I log it out, it continually comes out as false. however, when i have mocked it in my test like so:
const myMock = (myModule.myFunc = jest.fn())
myMock.mockReturnValue(true)
so why is it still coming back as false when I log it from the index file? or is that not quite how mocking works?
I'm guessing that myModule is the object you imported, and then you set the mock function on that object. But in the myModule file you are referencing that function directly, not through a module reference, right?
The proper way would probably be to move myFunc out of myModule. But if you want to keep it there, then you are going to have to partially mock myModule:
jest.mock('./myModule', () => {
return {
...jest.requireActual('./myModule'),
myFunc: jest.fn()
}
})
But seriously consider moving myFunc out of myModule, because partial mocking is difficult and confusing.
One way I found to solve my issue was to use a class instead.
Here is a sudo example:
Implementation
export class Location {
getLocation() {
const environment = this.getEnvironmentVariable();
return environment === "1" ? "USA" : "GLOBAL";
}
getEnvironmentVariable() {
return process.env.REACT_APP_LOCATION;
}
}
Test
import { Location } from "./config";
test('location', () => {
const config = new Location();
jest.spyOn(config, "getEnvironmentVariable").mockReturnValue("1")
const location = config.getLocation();
expect(location).toBe("USA");
});
When testing a module that has a dependency in a different file and assigning that module to be a jest.mock, TypeScript gives an error that the method mockReturnThisOnce (or any other jest.mock method) does not exist on the dependency, this is because it is previously typed.
What is the proper way to get TypeScript to inherit the types from jest.mock?
Here is a quick example.
Dependency
const myDep = (name: string) => name;
export default myDep;
test.ts
import * as dep from '../depenendency';
jest.mock('../dependency');
it('should do what I need', () => {
//this throws ts error
// Property mockReturnValueOnce does not exist on type (name: string)....
dep.default.mockReturnValueOnce('return')
}
I feel like this is a very common use case and not sure how to properly type this.
You can use type casting and your test.ts should look like this:
import * as dep from '../dependency';
jest.mock('../dependency');
const mockedDependency = <jest.Mock<typeof dep.default>>dep.default;
it('should do what I need', () => {
//this throws ts error
// Property mockReturnValueOnce does not exist on type (name: string)....
mockedDependency.mockReturnValueOnce('return');
});
TS transpiler is not aware that jest.mock('../dependency'); changes type of dep thus you have to use type casting. As imported dep is not a type definition you have to get its type with typeof dep.default.
Here are some other useful patterns I've found during my work with Jest and TS
When imported element is a class then you don't have to use typeof for example:
import { SomeClass } from './SomeClass';
jest.mock('./SomeClass');
const mockedClass = <jest.Mock<SomeClass>>SomeClass;
This solution is also useful when you have to mock some node native modules:
import { existsSync } from 'fs';
jest.mock('fs');
const mockedExistsSync = <jest.Mock<typeof existsSync>>existsSync;
In case you don't want to use jest automatic mock and prefer create manual one
import TestedClass from './TestedClass';
import TestedClassDependency from './TestedClassDependency';
const testedClassDependencyMock = jest.fn<TestedClassDependency>(() => ({
// implementation
}));
it('Should throw an error when calling playSomethingCool', () => {
const testedClass = new TestedClass(testedClassDependencyMock());
});
testedClassDependencyMock() creates mocked object instance
TestedClassDependency can be either class or type or interface
Use the mocked helper
like explained here
// foo.spec.ts
import { foo } from './foo'
jest.mock('./foo')
// here the whole foo var is mocked deeply
const mockedFoo = jest.mocked(foo, true)
test('deep', () => {
// there will be no TS error here, and you'll have completion in modern IDEs
mockedFoo.a.b.c.hello('me')
// same here
expect(mockedFoo.a.b.c.hello.mock.calls).toHaveLength(1)
})
test('direct', () => {
foo.name()
// here only foo.name is mocked (or its methods if it's an object)
expect(jest.mocked(foo.name).mock.calls).toHaveLength(1)
})
There are two solutions tested for TypeScript version 3.x and 4.x, both are casting desired function
1) Use jest.MockedFunction
import * as dep from './dependency';
jest.mock('./dependency');
const mockMyFunction = dep.myFunction as jest.MockedFunction<typeof dep.myFunction>;
2) Use jest.Mock
import * as dep from './dependency';
jest.mock('./dependency');
const mockMyFunction = dep.default as jest.Mock;
There is no difference between these two solutions. The second one is shorter and I would therefore suggest using that one.
Both casting solutions allows to call any jest mock function on mockMyFunction like mockReturnValue or mockResolvedValue
https://jestjs.io/docs/en/mock-function-api.html
mockMyFunction.mockReturnValue('value');
mockMyFunction can be used normally for expect
expect(mockMyFunction).toHaveBeenCalledTimes(1);
I use the pattern from #types/jest/index.d.ts just above the type def for Mocked (line 515):
import { Api } from "../api";
jest.mock("../api");
const myApi: jest.Mocked<Api> = new Api() as any;
myApi.myApiMethod.mockImplementation(() => "test");
Cast as jest.Mock
Simply casting the function to jest.Mock should do the trick:
(dep.default as jest.Mock).mockReturnValueOnce('return')
Use as jest.Mock and nothing else
The most concise way of mocking a module exported as default in ts-jest that I can think of really boils down to casting the module as jest.Mock.
Code:
import myDep from '../dependency' // No `* as` here
jest.mock('../dependency')
it('does what I need', () => {
// Only diff with pure JavaScript is the presence of `as jest.Mock`
(myDep as jest.Mock).mockReturnValueOnce('return')
// Call function that calls the mocked module here
// Notice there's no reference to `.default` below
expect(myDep).toHaveBeenCalled()
})
Benefits:
does not require referring to the default property anywhere in the test code - you reference the actual exported function name instead,
you can use the same technique for mocking named exports,
no * as in the import statement,
no complex casting using the typeof keyword,
no extra dependencies like mocked.
The latest jest allows you to do this very easily with jest.mocked
import * as dep from '../dependency';
jest.mock('../dependency');
const mockedDependency = jest.mocked(dep);
it('should do what I need', () => {
mockedDependency.mockReturnValueOnce('return');
});
Here's what I did with jest#24.8.0 and ts-jest#24.0.2:
source:
class OAuth {
static isLogIn() {
// return true/false;
}
static getOAuthService() {
// ...
}
}
test:
import { OAuth } from '../src/to/the/OAuth'
jest.mock('../src/utils/OAuth', () => ({
OAuth: class {
public static getOAuthService() {
return {
getAuthorizationUrl() {
return '';
}
};
}
}
}));
describe('createMeeting', () => {
test('should call conferenceLoginBuild when not login', () => {
OAuth.isLogIn = jest.fn().mockImplementationOnce(() => {
return false;
});
// Other tests
});
});
This is how to mock a non-default class and it's static methods:
jest.mock('../src/to/the/OAuth', () => ({
OAuth: class {
public static getOAuthService() {
return {
getAuthorizationUrl() {
return '';
}
};
}
}
}));
Here should be some type conversion from the type of your class to jest.MockedClass or something like that. But it always ends up with errors. So I just used it directly, and it worked.
test('Some test', () => {
OAuth.isLogIn = jest.fn().mockImplementationOnce(() => {
return false;
});
});
But, if it's a function, you can mock it and do the type conversation.
jest.mock('../src/to/the/Conference', () => ({
conferenceSuccessDataBuild: jest.fn(),
conferenceLoginBuild: jest.fn()
}));
const mockedConferenceLoginBuild = conferenceLoginBuild as
jest.MockedFunction<
typeof conferenceLoginBuild
>;
const mockedConferenceSuccessDataBuild = conferenceSuccessDataBuild as
jest.MockedFunction<
typeof conferenceSuccessDataBuild
>;
As of Jest 24.9.0 here is how you can mock and correctly type both your Class/Object/function and Jest properties.
jest.MockedFunction
jest.MockedClass
What we would like for a typed mock is that the mocked object type contains the union of the mocked object type and the type of Jest mocks.
import foo from 'foo';
jest.mock('foo');
const mockedFoo = foo as jest.MockedFunction<typeof foo>;
// or: const mockedFooClass = foo as jest.MockedClass<typeof FooClass>;
mockedFoo.mockResolvedValue('mockResult');
// Or:
(mockedFoo.getSomething as jest.MockedFunction<typeof mockedFoo.getSomething>).mockResolvedValue('mockResult');
As you can see, you can either manually cast what you need or you'll need something to traverse all foo's properties/methods to type/cast everything.
To do that (deep mock types) you can use jest.mocked() introduced in Jest 27.4.0
import foo from 'foo';
jest.mock('foo');
const mockedFoo = jest.mocked(foo, true);
mockedFoo.mockImplementation() // correctly typed
mockedFoo.getSomething.mockImplementation() // also correctly typed
I have found this in #types/jest:
/**
* Wrap a function with mock definitions
*
* #example
*
* import { myFunction } from "./library";
* jest.mock("./library");
*
* const mockMyFunction = myFunction as jest.MockedFunction<typeof myFunction>;
* expect(mockMyFunction.mock.calls[0][0]).toBe(42);
*/
Note: When you do const mockMyFunction = myFunction and then something like mockFunction.mockReturnValue('foo'), you're a changing myFunction as well.
Source: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/jest/index.d.ts#L1089
The top rated solution by Artur Górski does not work with the last TS and Jest.
Use MockedClass
import SoundPlayer from '../sound-player';
jest.mock('../sound-player'); // SoundPlayer is now a mock constructor
const SoundPlayerMock = SoundPlayer as jest.MockedClass<typeof SoundPlayer>;
A recent library solves this problem with a babel plugin: https://github.com/userlike/joke
Example:
import { mock, mockSome } from 'userlike/joke';
const dep = mock(import('./dependency'));
// You can partially mock a module too, completely typesafe!
// thisIsAMock has mock related methods
// thisIsReal does not have mock related methods
const { thisIsAMock, thisIsReal } = mockSome(import('./dependency2'), () => ({
thisIsAMock: jest.fn()
}));
it('should do what I need', () => {
dep.mockReturnValueOnce('return');
}
Be aware that dep and mockReturnValueOnce are fully type safe. On top, tsserver is aware that depencency was imported and was assigned to dep so all automatic refactorings that tsserver supports will work too.
Note: I maintain the library.
This is ugly, and in fact getting away from this ugliness is why I even looked at this question, but to get strong typing from a module mock, you can do something like this:
const myDep = (require('./dependency') as import('./__mocks__/dependency')).default;
jest.mock('./dependency');
Make sure you require './dependency' rather than the mock directly, or you will get two different instantiations.
For me this was enough:
let itemQ: queueItemType
jest.mock('../dependency/queue', () => {
return {
add: async (item: queueItemType, ..._args: any) => {
// then we can use the item that would be pushed to the queue in our tests
itemQ = item
return new Promise(resolve => {
resolve('Mocked')
})
},
}
})
Then, whenever the add method is called it will execute this code above instead of pushing it to the queue, in this case.
With TypeScript 2.8 we can do like this with ReturnType:
import * as dep from "./depenendency"
jest.mock("./dependency")
const mockedDependency = <jest.Mock<ReturnType<typeof dep.default>>>dep.default
it("should do what I need", () => {
mockedDependency.mockReturnValueOnce("return")
})
I've read about the revealing prototype pattern. I've created a function that returns an object.
'use strict'
exports.testPrototype = (function() {
function testMe() {
return true
}
return {
testMe: testMe
}
})()
console.log(exports.testPrototype.testMe())
const test = Object.create(exports.testPrototype)
console.log(test.testMe())
The output of console.log is true on both calls.
The trouble is, I can't get Mocha to test it.
Here is my test:
const expect = require('expect')
const testPrototype = require('../testPrototype.js')
describe('testPrototype',() => {
it('Should return true', ()=> {
const test = Object.create(testPrototype)
expect(test.testMe()).toEqual(true)
})
})
When I run the test, I get the following error:
1) testPrototype Should return true:
TypeError: testPrototype.testMe is not a function
at Context.<anonymous> (testPrototype.spec.js:7:24)
I've been struggling with this all day. Why isn't the test working? Have I missed something simple?
Your module is adding a property called testProperty to the exports object:
exports.testPrototype = ...
But the Mocha test file is using the exports object itself, and not that particular property:
const testPrototype = require('../testPrototype.js')
Instead, you need to explicitly use the property:
const testPrototype = require('../testPrototype.js').testPrototype
I am encountering a behaviour that I can't explain.
Depending on using import or require, this test successes (import) or fails (require).
In case of fail, I have the following error:
PhantomJS 2.1.1 (Windows 7 0.0.0) immutability a number is immutable FAILED undefined is not a constructor (evaluating 'expect((0, _immutable.List)([1])).toEqualImmutable((0, _immutable.List)([1]))')
Here is the code :
import { Map, List } from 'immutable';
const expect = require("expect");
// import expectImmutable from "expect-immutable";
const expectImmutable = require("expect-immutable");
expect.extend(expectImmutable);
describe("immutability", () => {
describe("a number", () => {
function increment(currentState) {
return currentState + 1;
}
it("is immutable", () => {
expect(List([1])).toEqualImmutable(List([1]));
expect(Map({ a: 1 })).toEqualImmutable(Map({ a: 1 }));
let state = 42;
let nextState = increment(state);
expect(List([nextState])).toEqualImmutable(List([43]));
expect(List([state])).toEqualImmutable(List([42]));
});
});
});
Does anyone have an explanation of what is happening behind the scene?
Thanks #zerkms and #maioman your advices have really helped.
You were right #maioman, It was due to the export syntax used by the library.
es6 :
export default foo;
require :
module.exports = foo;
So when needed to require an es6 written export, we should handle the default keyword.
require("foo").default;